U.S. patent number 4,725,826 [Application Number 07/004,076] was granted by the patent office on 1988-02-16 for manipulator grip slip sensor.
Invention is credited to Bryan D. Hunter.
United States Patent |
4,725,826 |
Hunter |
February 16, 1988 |
Manipulator grip slip sensor
Abstract
An optical-electromechanical transducer to warn of slippage of a
workpiece clamped within the manipulator grips of a robot.
Inventors: |
Hunter; Bryan D. (Lansing,
MI) |
Family
ID: |
21709018 |
Appl.
No.: |
07/004,076 |
Filed: |
January 16, 1987 |
Current U.S.
Class: |
340/679;
294/907 |
Current CPC
Class: |
B25J
13/083 (20130101); B25J 19/021 (20130101); Y10S
294/907 (20130101) |
Current International
Class: |
B25J
13/08 (20060101); B25J 19/02 (20060101); G08B
021/00 () |
Field of
Search: |
;340/679,686,540,674,407
;901/46-47,32,33,35,39 ;294/907,106 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rowland; James L.
Assistant Examiner: Mullen, Jr.; Thomas J.
Attorney, Agent or Firm: Lyon; Lyman R.
Claims
I claim:
1. A slip sensor for detecting movement of a workpiece relative to
a grip surface of a manipulator comprising
a spherical member having a modeled surface of randomly variable
reflectivity;
means on said manipulator for supporting said spherical member for
rotation and reciprocal movement toward and away from said grip
surface;
resilient means for biasing said spherical member into engagement
with said workpiece;
a light source for projecting a light beam onto the modeled surface
of said spherical member;
a light detector positioned in the path of a light beam reflected
from said spherical member, said light detector generating an
output proportional to the intensity of reflected light striking
said light detector;
a comparator connected to said light detector; and
signaling means, connected to said comparator and operable upon the
occurrence of a change in the output from said light detector.
2. The slip sensor of claim 1 wherein said rotatable member
comprises a hollow sphere.
3. A slip sensor comprising
a spherical rotatable member having a modeled surface of
continuously variable reflectivity for frictional engagement with a
workpiece clamped between a plurality of robot manipulator grip
surfaces;
a bushing affixed to one of said manipulator grips supporting said
spherical member for movement along the line normal to the surface
of said manipulator grip;
resilient means for biasing said spherical member into engagement
with the workpiece;
a light source for producing a light beam reflected off of the
modeled surface of said spherical member;
a light detector positioned generally in the path of said reflected
light beam, said light detector generating an electrical signal
proportional to the pattern and intensity of the reflected light
beam striking said light detector;
remote switching means;
signaling means for indicating workpiece slippage;
power output amplifier means for activating said signaling means;
and
comparator means for receiving the electrical signal generated by
said light detector and for comparing said electrical signal to a
reference value, said reference value being equal to the electrical
signal at the moment of activation of said remote switching
element, deviation of the sensed electrical signal from said
reference value generating a second electrical signal potential
sufficient to cause said power output amplifier means to activate
said signaling means.
4. The slip sensor of claim 3 including a hollow spherical
rotatable member.
Description
BACKGROUND OF THE INVENTION
The invention relates to an optical-electromechanical transducer to
monitor workpiece motion relative to the grip surface of a robot
manipulator.
The sensor assembly of the instant invention is an improvement over
the "Manipulator with Electro-Mechanical Transducer Means"
disclosed in U.S. Pat. No. 3,171,549 issued Mar. 2, 1965, and the
"Material Handling Apparatus" disclosed in U.S. Pat. No. 3,904,234
issued Sept. 9, 1975. While a slip sensor constructed in accordance
with the teachings of these patents serves a useful purpose, it is
not suitable for use in applications demanding decreased transducer
assembly size and weight, increased sensitivity, and increased
resistance to failure upon introduction of minute foreign particles
into the sensor environment.
Specifically, the sensor disclosed in the "549" patent comprises a
large number of elements, including a piston encompassing a
plurality of solid contiguously coupled spheres, substantially
increasing the overall size and weight of the assembly. The
diameter of both driving and driven spheres must be small in
comparison to the overall size of the sensor assembly, and, hence,
the use of larger, more sensitive spheres is prohibited. Moreover,
the sensor employs a high-rate spring to force engagement of these
adjacent spheres, as the low coefficient of friction incident to
the metal-to-metal sphere contact requires greater normal force in
order to produce adequate proportional rotational response of the
driven sphere in relation to the driving sphere. The use of a
high-rate spring additionally produces greater frictional contact
between the spheres and their bearing surfaces, producing a greater
resistance to rotation, thereby reducing transducer sensitivity.
The introduction of foreign particulate into the transducer
environment exacerbates this frictional contact, as the surface of
the driving sphere will carry the particles inside the transducer
assembly, causing seizing of the assembly and reducing transducer
sensitivity, reliability and responsiveness.
The "234" patent teaches an assembly comprising a light source, a
light detector and a movable vane or shutter placed between the
light source and light detector, with the vane's position varying
proportionately with the amount of workpiece displacement. However,
this "shutter technique" cannot accommodate a spherical rotatable
member having a continuous, nonperforated surface, and is therefore
not suitable for applications requiring the sensing of planar, as
opposed to linear, slip in the absence of a second,
orthogonally-oriented linear slip sensor.
SUMMARY OF THE INVENTION
It is the purpose of this invention to provide a single transducer
for a robot manipulator grip whereby planar workpiece movement
relative to the gripping surface may be detected, the transducer
being small, lightweight, and highly sensitive, responsive and
reliable.
Another purpose of the invention is to provide a transducer that is
recalibrated by operation of a remote switching element so that,
upon stabilization of the workpiece within the manipulator grip,
the operator may reset the sensor to allow for the signaling of any
additional workpiece movement.
The invention accomplishes these purposes by means of improvements
in both transducer structure and in the means employed to detect
workpiece movement relative to the grip surface. Specifically, the
transducer of the instant invention is a sensor assembly comprising
a spherical rotatable member having a modeled surface pattern of
varying reflectiveness, a bushing to provide for free-floating
containment of the spherical member, low-rate resilient biasing
means to maintain the requisite frictional contact between the
spherical member and the workpiece, a light source, a light
detector, a comparator to monitor changes in the amount of light
received by the light detector, and signaling means, whereby the
operator may be alerted to the workpiece slippage.
The light beam produced by the light source is reflected off of the
modeled surface of the spherical member. The light detector is
placed so that the reflected light beam falls upon the detector's
photosensitive surface. The detector thereby generates an output,
the magnitude of which varies with the pattern and intensity of the
reflected light beam. As the spherical member is rotated by
workpiece movement, the light beam strikes an area on the surface
of the sphere having a different reflective characteristic,
producing a change in the pattern and/or intensity of the light
beam reaching the detector. This in turn produces a change in the
magnitude of the output of the detector. Comparator means is used
to sample the output of the detector. When a change in the output
occurs, workpiece slippage is indicated, and the comparator
triggers a visual and/or audible warning signal to the operator.
The signal may also be fed into an automatic control system to
effect an instantaneous increase in grip pressure, thereby
preventing further workpiece slippage.
A remote switching element allows the operator or automatic control
system to re-enable the sensor and to establish the time at which
the reference used by the comparator is measured. A timing device
may be used to automatically set the reference value after a
specified interval subsequent to manipulator grip clamping
movement.
The reduced assembly size allows for placement of the sensor into
smaller, lighter manipulator grips physically incapable of
receiving the larger, heavier sensors embodying the prior art. By
reducing the number of elements, the overall size and weight of the
transducer assembly is reduced, even where a larger diameter
spherical member is employed. The use of a larger spherical member
provides greater transducer sensitivity, as a larger modeled
surface produces a more narrow, brighter beam of reflected light.
The sensitivity is further increased through the use of low-rate
resilient biasing means to force engagement between the spherical
member and the workpiece, whereby the frictional resistance to
rotation produced by sphere-bushing contact is reduced, promoting a
truer free-spinning condition. The use of a hollow spherical
rotatable member allows for greater transducer responsiveness due
to the sphere's lower inertial mass.
The elimination of full bearing contacts in favor of a cylindrical
bushing allowing for free movement along the line normal to the
manipulator grip surface greatly reduces the likelihood of seizing
of the assembly upon introduction of foreign particulate into the
sensor environment. Moreover, transducer response is not adversely
affected by the adherence of foreign particles to the reflecting
surface of the sphere, as it is the change in reflectivity produced
by sphere rotation, rather than any given value of reflectivity,
that triggers the warning. The use of reflected light allows for
greater flexibility in choosing the mounting locations of the light
source and light detector elements, as well as for less stringent
alignment requirements during installation since the sensor will
work notwithstanding slight misalignment of these elements.
Additionally, the reliability of a transducer employing this
sensing technique is increased as the sensor components are not
readily thrown from workable alignment when place in actual
service.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view in perspective of a robot device having a
manipulator grip containing a slip sensor constructed in accordance
with the invention; and
FIG. 2 is an enlarged view, partly cut out, partly in section, of
the lower jaw of the manipulator grip.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
FIG. 1 shows a robot device 10 having an articulated manipulator
grip 12 comprising a lower jaw 14 and an upper jaw 16. A workpiece
18 is shown clamped between the lower jaw 14 and the upper jaw 16.
An enlarged view of the lower jaw 14 is illustrated in FIG. 2. The
lower jaw 14 houses a slip sensor assembly 30 constructed in
accordance with the instant invention. A circular aperture 20 is
formed in the grip surface 22 of the lower jaw 14 to accept the
slight protuberance of a hollow thin-walled spherical rotatable
member 32, such that the spherical member 32 may frictionally
engage with the workpiece 18 clamped between the lower jaw 14 and
the upper jaw 16. The hollow thin-walled spherical member 32,
having a diameter slightly greater than that of the circular
aperture 20, is provided with a modeled surface pattern of
continuously varying reflectivity by such means as etching,
rolling, painting or cutting.
A cylindrical bushing 34, characteristically formed of a polymeric
substance, is affixed by conventional means to the underside of the
grip surface 22 of the lower jaw 14. The cylindrical bushing 34
allows for the containment of the spherical member 32 in the plane
parallel to that of the grip surface 22 while providing freedom of
movement of the spherical member 32 along the line normal to the
plane of the grip surface 22. An elongated rigid member 36 is
attached to the cylindrical bushing 34 at the end opposite that
which is attached to the underside of the grip surface 22, with the
free end of the elongated rigid member 36 extending in the
direction normal to and away from the plane of the grip surface 22.
An elongated resilient biasing member 38 is orthogonally attached
to the free end of the elongated rigid member 36 so as to extend
radially inward across the end of the cylindrical bushing 34. A low
friction shell 40 having a concave inner surface defined by a
radius equal to that of the spherical member 32 is attached to the
radially inward end of the elongated resilient biasing member 38 so
as to accept the spherical member 32.
The diameter of the spherical member 32, and the length of the
cylindrical bushing 34 and the elongated rigid member 36, re such
that the spherical member 32 protrudes beyond the grip surface 22
when the elongated resilient biasing member 38 is in an unloaded
state. The circular aperture 20 acts as a retainer so as to prevent
the spherical member 32 from falling through the grip surface 22
and free of the sensor assembly 30.
A light source 42, herein disposed of as a light emitting diode
(LED), is mounted within the lower jaw 14 so as to project its
light beam onto the reflective surface of the spherical member 32.
A light detector 44, herein disposed of as a photocell of
conventional design, is also mounted within the lower jaw 14 so as
to be placed generally in the path of the reflected light beam,
with the light beam striking the photosensitive surface 46 of the
photocell 44. A lens 60 may be employed to focus the light beam
generated by the LED 42 onto the surface of the spherical member
32. Similarly, a second lens 62 may be employed to collect the
reflected light beam for use by the photocell 44.
As shown schematically in FIG. 1, the lead wires 48 from the
photocell 44 are connected to a remotely positioned control unit
58. FIG. 2 schematically shows the control unit 58 comprising a
comparator 50 of conventional design; a remote switching element
52, herein disposed of as a momentary-on push button; a power
output amplifier 54; and a signaling device 56, herein disposed of
as a signal lamp.
The photocell 44 generates a potential proportional to the pattern
and intensity of the light beam striking its photosensitive surface
46. The potential is carried by the lead wires 48 to the comparator
50, which compares the potential instantaneously generated by the
light detector 44 with a reference value, the reference value being
the potential generated at the moment of last activation of the
push button 52. When the value of the instantaneous potential
differs from the reference value, the comparator 50 generates a
second potential which, after amplification by the power output
amplifier 54, activates the signal lamp 56.
As the workpiece 18 is clamped between the lower jaw 14 and the
upper jaw 16 of the manipulator grip 12, the workpiece 18 is first
brought into engagement with the spherical member 32, and then with
the grip surface 22. The spherical member 32 is displaced back
through the circular aperture 20, deflecting the elongated
resilient biasing member 38 away from the grip surface 22. The
deflected elongated resilient biasing member 38 exerts a
reactionary force back against the spherical member 32 as
transmitted through the low-friction shell 40, so as to maintain
sufficient frictional contact between the spherical member 32 and
the workpiece 18. The operator depresses the push button 52
subsequent to the clamping movement of the manipulator jaws 14 and
16, thereby setting the initial reference value to be used by the
comparator 50.
The movement of the workpiece 18 relative to the grip surface 22 of
the lower jaw 14 produces a rotation of the spherical member 32. As
the spherical member 32 rotates, the light beam generated by the
LED 42 falls upon a different area on the surface of the spherical
member 32, producing a different pattern and intensity of reflected
light striking the photosensitive surface 46 of the photocell 44.
The potential generated by the photocell 44 therefore differs from
the reference value, and the comparator 50 generates a second
output potential which, after amplification by the power output
amplifier 54, activates the signal lamp 56. The operator is thereby
alerted of workpiece slippage, and may increase the grip pressure
to prevent further movement of the workpiece 18 relative to the
grip surface 22. The operator depresses the push button 52 to reset
the reference value used by the comparator 50 and re-enable the
sensor.
It is to be noted that the invention contemplates the use of fiber
optics to effect a further reduction in the size and weight of the
transducer assembly while increasing its reliability. For example,
optical fibers may be substituted for the wires 48, with the LED 42
and the photocell 44 being placed within the control unit 58. The
effects of electromagnetic interference on the system are thereby
reduced; any electromagnetic interference produced by the wires
themselves is eliminated; and there is no longer the possible
safety hazard of a broken wire generating a spark. Additionally,
such use of fiber optics allows for the common management of many
serially-linked slip sensors, as where the light output of one
sensor becomes the light input of the next sensor. Purely optical
comparator means may also be employed in conjunction with the use
of fiber optics.
The invention additionally contemplates the use of an automatic
control system to automate one or more of the following operations:
initiating clamping movement of the manipulator grip jaws;
activating the remote switching element, whereby the initial
comparator reference value is determined; incorporating internal
signaling means whereby the output of the comparator means triggers
the application of additional grip pressure; and resetting the
sensor upon the application of additional grip pressure.
While the preferred embodiment of the invention has been disclosed,
it should be appreciated that the invention is susceptible of
modification without departing from the spirit of the invention or
the scope of the subjoined claims.
* * * * *